Aromatics processing catalyst system

ABSTRACT

This disclosure relates to a catalyst system adapted for processing aromatic feedstreams comprising C 9 + aromatic feedstock to produce at least one xylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application No. 61/264,891, filed Nov. 30, 2009, the disclosure of which is incorporated by reference in its entirety.

FIELD

The disclosure relates to the catalyst system for the production of xylenes.

BACKGROUND

Paraxylene (PX), which is separated from metaxylene (MX) and orthoxylene (OX) on a commercial scale typically by subzero crystallization separation or simulated moving bed adsorptive separation, has widespread applications in many fields such as chemical fiber, synthetic resin, pesticide, medicine, plastics, and so forth. The use of disproportionation of toluene, or the disproportionation and transalkylation of toluene and C9+ aromatics to produce benzene and C8 isomers (including xylenes and ethyl benzene) are effective routes for increasing the output of PX. Examples of these methods are described, for instance, in U.S. Pat. No. 6,528,695.

A source of xylene is catalytic reformate. Catalytic reformate is typically prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. A C6 to C8 fraction can be separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (collectively “BTX”), along with ethylbenzene. The separation of this mixture into its constituent parts to upgrade the value thereof has been and still is the subject of continuous research.

Xylene is also available from the transalkylation of C9+ aromatics and toluene over noble metal-containing zeolite catalysts. During the transalkylation of C9+ aromatics and toluene to high value petrochemical products, by-products, such as saturated compounds, are typically produced. These by-products can boil in the same temperature range as the desired aromatic products, making separation of the desired products at high purity levels difficult. For example, a commercial benzene product may need a purity of 99.85 wt % or higher. However, initial benzene purity after distillation of a transalkylation reaction product is typically only 99.2 wt % to 99.5 wt % due to the presence of co-boilers, such as methylcyclopentane, cyclohexane, 2,3-dimethylpentane, dimethylcyclopentane and 3-methylhexane. Therefore, an additional extraction step is usually required to further improve benzene product purity to the desired level.

One solution to the problem of the production of benzene co-boilers during the transalkylation of heavy aromatics is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. A benzene product having a purity of at least 99.85% may be obtained by distilling the benzene from the transalkylation reaction product, without the need for an additional extraction step. According to the '651 patent, the second catalyst composition comprises up to 20 wt. % of the total weight of the first and second catalyst compositions.

U.S. Pat. No. 5,905,051 discloses a process for converting a hydrocarbon stream such as, for example, a C₉+ aromatic compound to C₆ to C₈ aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product.

U.S. Pat. No. 5,030,787 discloses an improved disproportionation/transalkylation process. The improved process of this invention is conducted such that transalkylation of a C9+ aromatics feedstock, or disproportionation of a feedstock containing toluene and C9+ aromatic(s), is carried out in the vapor-phase by containing said feedstock in a reaction zone with a catalyst comprising a zeolite possessing a Constraint Index, as defined below, of from 1 to about 3 and preferably which has been hydrogen, hydrogen precursor and/or non-noble Group VIII metal exchanged, thermally treated and/or hydrothermally treated, under conditions effective to convert such feedstock to a product containing substantial quantities of C6-C8 aromatic compounds, e.g. benzene and xylene(s), especially the latter. The product effluent is separated and distilled to remove the desired products. If desired, any unreacted material(s), e.g., toluene and/or C₉+ compound(s), can be recycled.

U.S. Pat. No. 5,030,787 discloses a transalkylation process to convert a heavy aromatics feed to lighter aromatics products, such as benzene, toluene and xylenes by contacting a C₉+ aromatics fraction and benzene and/or toluene over a catalyst comprising a zeolite, such as ZSM-12, and a hydrogenation component, preferably platinum. The catalyst, with hydrogenation component, is treated to reduce aromatics loss. Treatment includes exposure to steam and/or sulfur after incorporation of the hydrogenation component. For additional stability and aromatics retention, the steamed and/or sulfur treated catalyst is sulfided by cofeeding a source of sulfur. In a further embodiment of the invention, a low hydrogen partial pressure is employed to retain aromatics.

U.S. Pat. No. 7,148,391 discloses a single stage catalyst system comprising at least two different molecular sieves that exhibits enhanced activity for the removal of ethyl-group containing aromatic compounds in C9+ aromatic feeds without overall reduction in the conversion of the C9+ feed to useful compounds, such as xylenes.

U.S. Pat. No. 7,485,763 teaches a process for converting C9+ aromatic hydrocarbons to lighter aromatic products, comprising the step of contacting a feed comprising C9+ aromatic hydrocarbons under transalkylation reaction conditions with a catalyst composition comprising: (i) a first molecular sieve selected from the group consisting of ZSM-12, mordenite and a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07; and (ii) a second molecular sieve having a constraint index ranging from 3 to 12, wherein at least the first molecular sieve has a hydrogenation component associated therewith and wherein the first and second molecular sieves are contained in the same catalyst bed, the C₉+ aromatic hydrocarbons being converted under said transalkylation reaction conditions to a reaction product containing xylene. In one embodiment, the first molecular sieve is ZSM-12 and the second molecular sieve is ZSM-5.

U.S. Pat. No. 7,553,791 teaches a process for the conversion of a feedstock containing C9+ aromatic hydrocarbons to produce a resulting product containing lighter aromatic products and less than about 0.5 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product, said process comprising contacting said feedstock under transalkylation reaction conditions with a catalyst composition comprising: (i) an acidity component having an alpha value of at least 300; and (ii) a hydrogenation component having hydrogenation activity of at least 300, the C₉+ aromatic hydrocarbons being converted under said transalkylation reaction conditions to a reaction product containing xylenes. Preferably, the aromatic product contains less than about 0.3 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product. More preferable, the aromatic product contains less than about 0.2 wt % of ethylbenzene based on the weight of C8 aromatics fraction of said resulting product. Preferably, the acidity component comprises a molecular sieve selected from the group consisting of one or more of a first molecular sieve having a MTW structure, a molecular sieve having a MOR structure, and a porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima (Å) at 12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07. More preferably, the catalyst comprises a molecular sieve ZSM-12. Alternatively, the porous crystalline inorganic oxide material is selected from the group consisting of one or more of MCM-22, PSH-3, SSZ-25, MCM-36, MCM-49 and MCM-56. In another embodiment, the catalyst comprises second molecular sieve having a constraint index ranging from 3 to 12. Preferably, the second molecular sieve is ZSM-5. Preferably, the catalyst comprises two molecular sieves, the first molecular sieve is ZSM-12, and the second molecular sieve is ZSM-5. Conveniently, the catalyst composition is particulate and the first and second molecular sieves are each contained in the same catalyst particles. Preferably, the hydrogenation component is selected from the group consisting of one or more of a Group VIIIB and Group VIIB metal. More preferably, the hydrogenation component is selected from the group consisting of one or more of rhenium, platinum, and palladium.

A catalyst system for the transalkylation of C9+ aromatics with C6-C7 aromatics is disclosed in U.S. Pat. No. 7,663,010. The catalyst system described therein comprises (a) a first catalyst comprising a molecular sieve having a Constraint Index in the range of 3-12 (e.g., a 10 MR molecular sieve, such as ZSM-5, ZSM-11, ZSM-22, and ZSM-23) and a metal catalyzing the saturation of the olefins formed by the dealkylation reactions and (b) a second catalyst comprising a molecular sieve having a Constraint Index in the range of less than 3 (e.g., a 12 MR molecular sieve, such as ZSM-12, MOR, zeolite beta, MCM-22 family molecular sieve) and optionally a metal which may be the same or different to the metal on the first catalyst.

Improving catalytic activity and stability are challenges for most of the catalytic transalkylation processes. A high activity catalyst normally requires less catalyst and/or less severe reaction conditions to manufacture the same amount of product, which means lower cost for production and high production efficiency. As the catalyst ages with increasing time on stream, higher temperatures are normally required to maintain constant conversion. When the maximum reactor temperature is reached, the catalyst needs to be replaced or regenerated. Depending on the feed composition, the cycle length varies from a few months to as long as a few years for a transalkylation catalyst. A catalyst having high stability normally requires less frequent regeneration or change-out and long time on stream, which translates to lower cost for production and high production efficiency.

The present inventor has now discovered that a catalyst system comprising two beds, a first bed comprising aluminosilicates having a MWW topology, e.g. MCM-22 and MCM-49, and containing a metal, and a second bed, in series with said first bed, comprising an aluminosilicate containing a 12 MR structure, e.g. MTW (ZSM-12), that may also contain a metal, allows for processing of heavy aromatic feeds at space velocities significantly higher than competing technologies. This provides a significant advantage in that higher throughput is possible in the transalkylation unit. In addition, it has been discovered that the use of this dual bed configuration results in lower aging rates for the catalysts, thereby extending cycle lengths.

As an optional embodiment of this invention, a third bed, located just prior to the exit of the reactor, may be used to crack non-aromatic molecules, e.g. cyclohexane, to meet benzene purity specifications. The catalyst used in this optional third bed is preferably one containing an MFI zeolite.

SUMMARY

The invention concerns a catalyst system comprising two beds, the first bed comprising aluminosilicates having an MWW topology, e.g. MCM-22 and MCM-49, and at least one metal selected from Groups 6-10 from the Periodic Table, and a second bed, in series and in fluid communication with the first bed, the second bed comprising an aluminosilicate containing a 12 MR structure, e.g. MTW (ZSM-12), and at least one metal selected from Groups 6-10 of the Periodic Table. The metal in the first bed may be the same or different from the metal in the second bed. Optionally, there is a third bed in series after said second bed and in fluid communication therewith, said third bed comprising a MFI zeolite.

The invention is also directed to a process comprising contacting a feedstream comprising benzene, toluene and C9+ aromatic compounds with said first bed and then said second bed, and then, optionally, said third bed, to produce a product comprising xylene. The product can also be characterized as having significantly reduced concentration of C9+ aromatic compounds when compared with the feedstream fed to the first, second, and optionally third bed.

The invention also concerns a reactor comprising said first and second beds and optionally said third bed.

In embodiments the invention is directed to a catalyst system comprising two beds in series, a first bed comprising aluminosilicates having a MWW topology, and also comprising a metal effective for hydrogenation of olefinic components, and a second bed comprising an aluminosilicate containing a 12 MR structure, that also comprises a metal effective for hydrogenation, and optionally a third bed comprising a zeolite having an MFI topology.

In embodiments, the process of the invention includes a step of contacting a feedstream comprising C6, C7 and C9+ aromatics compounds with the first bed, whereby C9+ components of the feed containing ethyl, propyl and butyl sidechains are dealkylated such that the ethyl, propyl and butyl groups on the aromatic products of the first bed are substantially reduced when compared with the feedstream, then contacting the product of the first bed with the catalyst in the second bed, whereby transalkylation occurs between C9+ aromatic components and the C6 and C7 aromatic components of the feedstream in order to provide a product of the second bed having an increased concentration in at least one xylene isomer when compared with the feedstream prior to contact with the first bed. Optionally, the product of the second bed is then contacted with the third bed, located just prior to the exit of the reactor, comprising an MFI zeolite, whereby non-aromatic molecules, e.g. cyclohexane, are cracked and/or benzene purity specifications are improved.

The invention also concerns the production of xylene comprising the steps of (a) contacting a C9+ aromatic feedstock, hydrogen and a C6-C7 aromatic feedstock with a first catalyst comprising 0.01 to 5 wt. % of at least one source of a first metal element of Groups 6-10 and a first molecular sieve having a zeolite structure type of MWW under first conditions to form a first product, wherein said first conditions are selected such that said first product is substantially free of olefinic components and said first product preferably contains at least 30 wt. % less ethyl-aromatic compounds and at least 60 wt. % less propyl-aromatic compounds than the C9+ aromatic feedstock; then (b) contacting at least a portion of said first product with a second catalyst comprising 0 to 5 wt. % of at least one source of a second metal element of Groups 6-10 and a second molecular sieve having a Constraint Index less than 3 under second conditions, wherein said second conditions are sufficient to transalkylate at least a portion of the C9+ aromatic compounds in said C9+ aromatic feedstock with at least a portion of the C6-C7 aromatic compounds in said C6-C7 aromatic feedstock to form a second product comprising xylene, wherein said second conditions are selected such that said second product is substantially free of olefinic components and a xylene yield is preferably in the range of 20 to 50 wt. %, and wherein said second product contains at least 50 wt. % less ethyl-aromatic compounds and at least 70 wt. % less propyl-aromatic compounds than the C9+ aromatic feedstock; and then (c) recovering xylene.

In an embodiment, there is also a step, preferably after step (b) and prior to the step of recovery of xylene, of contacting the feedstream, such as from step (b) with a catalyst useful for cracking non-aromatic molecules, e.g. cyclohexane. The catalyst used in this optional third bed is preferably one containing an MFI zeolite.

In a preferred embodiment, the first metal element and the second metal element are at least one of Pt, Re, Ir, and Pd.

In another preferred embodiment, the weight ratio of the first catalyst over the second catalyst is in the range of 5:95 to 75:25, preferably in the range of 20:80 and 50:50.

In some embodiments, the process of this disclosure further comprises a step of adjusting the flowrate of the C₉+ aromatic feedstock and the flowrate of the C₆-C₇ aromatic feedstock so that the combined aromatic feedstock having a methyl to single aromatic ring molar ratio in the range between 0.5 to 4, preferably between 1.0 to 2.5.

In some aspects, the first conditions are selected such that the first product is substantially free of olefinic components and the first product contains at least 50 wt. % less ethyl-aromatic compounds and at least 70 wt. % less propyl-aromatic compounds than the C₉+ aromatic feedstock and wherein the second conditions are selected such that the second product is substantially free of olefinic components and the xylene yield is in the range of 25 to 40 wt. %, and wherein the second product contains at least 70 wt. % less ethyl-aromatic compounds and at least 85 wt. % less propyl-aromatic compounds than the at least a portion of the first product.

In other aspects, the first molecular sieve has an alpha value in the range of 100 to 1500, preferably in the range of 200 to 500.

In some embodiments, where the C₉+ aromatic feedstock and/or the C₆-C₇ aromatic feedstock contains paraffinic compounds, the process further comprises a step of contacting the paraffinic compounds in the C₉+ aromatic feedstock and/or the C₆-C₇ aromatic feedstock with a third catalyst comprising a third molecular sieve having a Constraint Index in the range of 3-12 under cracking conditions sufficient to crack at least 50 wt. % of the paraffinic compounds.

In some embodiments, where the second product contains paraffinic compounds, the process further comprises a step of contacting the paraffinic compounds in the second product with a fourth catalyst comprising a fourth molecular sieve having a Constraint Index in the range of 3-12 under cracking conditions sufficient to crack at least 50 wt. % of the paraffinic compounds in the second product.

In some aspects, the first conditions comprise a temperature in the range of 100 to 1000° C., a pressure in the range of 790 to 7000 kPa-a (kilo-Pascal absolute), a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹, and wherein the second conditions comprise a temperature in the range of 100° C. to 1000° C., a pressure in the range of 790 to 7000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹.

In other aspects, the first conditions and the second conditions are selected such that the total ring-loss of the process is in the range of 0 to 3 wt. %.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a process for converting a feed comprising C₉+ aromatic hydrocarbons, hydrogen, and C₆-C₇ aromatic hydrocarbons to produce a product containing xylenes. The process involves contacting a C₉+ aromatic feedstock, hydrogen and a C₆-C₇ aromatic feedstock with a first catalyst comprising a first molecular sieve having a zeolite structure type of MWW and 0.01 to 5 wt. %, preferably 0.01 to 1 wt. % of at least one source of a first metal element of Groups 6-10 under first conditions to form a first product, wherein said first conditions are selected such that said first product is substantially free of olefinic components and said first product preferably contains at least 30 wt. % less ethyl-aromatic compounds and at least 60 wt. % less propyl-aromatic compounds than the C9+ aromatic feedstock; then contacting at least a portion of said first product with a second catalyst comprising a second molecular sieve having a Constraint Index less than 3 and 0 to 5 wt. %, preferably 0.01 to 1 wt. % of at least one source of a second metal element of Groups 6-10 and under second conditions, wherein said second conditions are sufficient to transalkylate at least a portion of the C9+ aromatic compounds in said C9+ aromatic feedstock with at least a portion of the C6-C7 aromatic compounds in said C6-C7 aromatic feedstock to form a second product comprising xylenes, wherein said second conditions are selected such that said second product is substantially free of olefinic components and a xylene yield is preferably in the range of 20 to 50 wt. %, and wherein said second product contains at least 50 wt. % less ethyl-aromatic compounds and at least 70 wt. % less propyl-aromatic compounds than the C9+ aromatic feedstock; and then recovering xylene.

As used in this specification, the term “framework type” is used in the sense described in the “Atlas of Zeolite Framework Types,” 2001.

As used herein, the numbering scheme for the Periodic Table Groups is used as in Chemical and Engineering News, 63(5), 27 (1985).

The MCM-22 family materials are well-known per se in the art and may also be characterized as porous crystalline inorganic oxide material having an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 3.57±0.07 and 3.42±0.07 Angstroms (either calcined or as-synthesized). The X-ray diffraction data used to characterize the molecular sieve are obtained by standard techniques using the K-alpha doublet of copper as the incident radiation and a diffractometer equipped with a scintillation counter and associated computer as the collection system. Materials belong to the MCM-22 family include MCM-22 (described in U.S. Pat. No. 4,954,325 and U.S. patent application Ser. No. 11/823,722), PSH-3 (described in U.S. Pat. No. 4,439,409), SSZ-25 (described in U.S. Pat. No. 4,826,667), ERB-1 (described in European Patent No. 0293032), ITQ-1 (described in U.S. Pat. No. 6,077,498), ITQ-2 (described in International Patent Publication No. WO97/17290), ITQ-30 (described in International Patent Publication No. WO2005118476), MCM-36 (described in U.S. Pat. No. 5,250,277), MCM-49 (described in U.S. Pat. No. 5,236,575), UZM-8 (described in U.S. Pat. No. 6,756,030), MCM-56 (described in U.S. Pat. No. 5,362,697), EMM-10-P (described in U.S. patent application Ser. No. 11/823,129), and EMM-10 (described in U.S. patent application Ser. Nos. 11/824,742 and 11/827,953). The entire contents of the patents are incorporated herein by reference.

The term “wppm” as used herein is defined as parts per million by weight.

As used herein, the term “substantially free” means less than 1 wt. %, preferably less than 0.1 wt. %. For example, that a product is substantially free of olefinic components means the product has less than 1 wt. %, preferably less than 0.1 wt. % olefinic components based on the total weight of the product.

The xylene yield, as used herein, is calculated by dividing the total weight of the xylene isomers (para-, meta-, and ortho-xylene) by the total weight of the product stream. The total weight of the xylene isomers can be calculated by multiplying the weight percentage of the xylene isomers, as determined by gas chromatography, by the total weight of the product stream.

The ring-loss, as used herein, is calculated by the following formula: Ring loss (%)=[1−(total moles of aromatic compounds in product/total moles of aromatic compounds in feed)]*100

The methyl over aromatic ring ratio is calculated by dividing the total moles of methyl group attaching to an aromatic ring in an aromatic feedstock over the total moles of single aromatic ring in the same aromatic feedstock.

The term “ethyl-aromatic compounds” means aromatic compounds having an ethyl group attached to the aromatic ring. The term “propyl-aromatic compounds” means aromatic compounds having a propyl group attached to the aromatic ring.

The ethyl content of the C₉+ aromatic feedstock is calculated by multiplying the molecular weight of C₂H₅ by the total mole fraction of aromatics having an ethyl group, where single ethyl substituted aromatics, e.g. 1,4-ethyltoluene are counted once, and di-substituted aromatic rings, e.g. 1,2-diethylbenzene, are counted twice.

The propyl content of the C₉+ aromatic feedstock is calculated by multiplying the molecular weight of C₃H₇ by the total mole fraction of aromatics having a propyl group, where single propyl substituted aromatics, e.g. n-propylbenzene are counted once, and di-substituted aromatic rings, e.g. 1,4-dipropylbenzene, are counted twice.

Weight of molecular sieve, weight of binder, weight of catalyst composition, weight ratio of molecular sieve over catalyst composition, weight ratio of the first catalyst over the second catalyst and weight ratio of binder over catalyst composition are calculated based on calcined weight (at 510° C. in air for 24 hours), i.e., the weight of the molecular sieve, the binder, and the catalyst composition are calculated based on the weight of the molecular sieve, the binder, and the catalyst composition after being calcined at 510° C. in air for twenty-four hours.

The term “aromatic” as used herein is to be understood in accordance with its art-recognized scope which includes alkyl substituted and unsubstituted mono- and polynuclear compounds.

The term “C_(n)” hydrocarbon wherein n is an positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbon having n number of carbon atom(s) per molecular. For example, C_(n) aromatics means an aromatic hydrocarbon having n number of carbon atom(s) per molecular. The term “C_(n)+” hydrocarbon wherein n is an positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbon having at least n number of carbon atom(s) per molecular. The term “C_(n)−” hydrocarbon wherein n is an positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein means a hydrocarbon having no more than n number of carbon atom(s) per molecular.

The term “C_(n) feedstock”, wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n) feedstock comprises greater than 50 wt. % of hydrocarbons having n number of carbon atom(s) per molecule. The term “C_(n)+ feedstock”, wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n)+ feedstock comprises greater than 50 wt. % of hydrocarbons having at least n number of carbon atom(s) per molecule. The term “C_(n)− feedstock” wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n)− feedstock comprises greater than 50 wt. % of hydrocarbons having no more than n number of carbon atom(s) per molecule. The term “C_(n) aromatic feedstock”, wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n) aromatic feedstock comprises greater than 50 wt. % of aromatic hydrocarbons having n number of carbon atom(s) per molecule. The term “C_(n)+ aromatic feedstock”, wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n)+ aromatic feedstock comprises greater than 50 wt. % of aromatic hydrocarbons having at least n number of carbon atom(s) per molecule. The term “C_(n)+ aromatic feedstock” wherein n is a positive integer, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, as used herein, means that the C_(n)+ aromatic feedstock comprises greater than 50 wt. % of aromatic hydrocarbons having no more than n number of carbon atom(s) per molecule.

Catalyst Composition

The catalyst composition used in the process of the disclosure comprises:

-   -   a. a first catalyst comprising a first molecular sieve having         0.01 to 5 wt. % of at least one source of a first metal element         of Groups 6-10 and having a structure type of MWW characterized         as porous crystalline inorganic oxide materials having an X-ray         diffraction pattern including d-spacing maxima at 12.4±0.25,         3.57±0.07 and 3.42±0.07 Angstroms; and     -   b. a second catalyst comprising a second molecular sieve having         0 to 5 wt. % of at least one source of a second metal element of         Groups 6-10 and a Constraint Index less than 3,         wherein the weight ratio of the first catalyst over the second         catalyst is in the range of 5:95 to 75:25 and wherein the first         catalyst is located in front of the second catalyst when they         are brought into contacting with the C₉+ aromatic feedstock and         the C₆-C₇ aromatic feedstock in the present of hydrogen.

The Constraint Index is a convenient measure of the extent to which an aluminosilicate or molecular sieve provides controlled access to molecules of varying sizes to its internal structure. For example, aluminosilicates which provide a highly restricted access to and egress from its internal structure have a high value for the constraint index, and aluminosilicates of this kind usually have pores of small size, e.g. less than 5 Angstroms. On the other hand, aluminosilicates which provide relatively free access to the internal aluminosilicate structure have a low value for the constraint index, and usually pores of large size. The method by which constraint index is determined is described fully in U.S. Pat. No. 4,016,218, which is incorporated herein by reference for the details of the method.

A molecular sieve having a Constraint Index of 3-12 (as defined in U.S. Pat. No. 4,016,218), includes ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-57 and ZSM-58. ZSM-5 is described in detail in U.S. Pat. No. 3,702,886 and Re. 29,948. ZSM-11 is described in detail in U.S. Pat. No. 3,709,979. ZSM-22 is described in U.S. Pat. Nos. 4,556,477 and 5,336,478. ZSM-23 is described in U.S. Pat. No. 4,076,842. ZSM-35 is described in U.S. Pat. No. 4,016,245. ZSM-48 is more particularly described in U.S. Pat. Nos. 4,234,231 and 4,375,573. ZSM-57 is described in U.S. Pat. No. 4,873,067. ZSM-58 is described in U.S. Pat. No. 4,698,217. The entire contents of all the above patent specifications are incorporated herein by reference.

A molecular sieve having a Constraint Index of less than 3 (as defined in U.S. Pat. No. 4,016,218), includes zeolite beta, zeolite Y, Ultrastable Y (USY), Dealuminized Y (Deal Y), mordenite, ZSM-3, ZSM-4, ZSM-12, ZSM-18, NU-87 and ZSM-20. Zeolite ZSM-4 is described in U.S. Pat. No. 3,923,636. Zeolite ZSM-12 is described in U.S. Pat. No. 3,832,449. Zeolite ZSM-20 is described in U.S. Pat. No. 3,972,983. Zeolite Beta is described in U.S. Pat. No. 3,308,069, and Re. No. 28,341. Low sodium Ultrastable Y molecular sieve (USY) is described in U.S. Pat. Nos. 3,293,192 and 3,449,070. Dealuminized Y zeolite (Deal Y) may be prepared by the method found in U.S. Pat. No. 3,442,795. Zeolite UHP-Y is described in U.S. Pat. No. 4,401,556. Rare earth exchanged Y (REY) is described in U.S. Pat. No. 3,524,820. Mordenite is a naturally occurring material but is also available in synthetic forms, such as TEA-mordenite (i.e., synthetic mordenite prepared from a reaction mixture comprising a tetraethylammonium directing agent). TEA-mordenite is disclosed in U.S. Pat. Nos. 3,766,093 and 3,894,104. The entire contents of all the above patent specifications are incorporated herein by reference.

In one embodiment, the first molecular sieve is a member of the MCM-22 family include MCM-22, PSH-3, SSZ-25, ERB-1, ITQ-1, ITQ-2, ITQ-30, MCM-36, MCM-49, UZM-8, MCM-56, EMM-10-P, and EMM-10. Examples of the second molecular sieve suitable for use are zeolite beta, zeolite Y, Ultrastable Y (USY), Dealuminized Y (Deal Y), mordenite, ZSM-3, ZSM-4, ZSM-12, ZSM-18, NU-87 and ZSM-20.

With regard to the molecular sieve having a Constraint Index of less than 3, ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449. Mordenite occurs naturally but may also be used in one of its synthetic forms, such as TEA-mordenite (i.e., synthetic mordenite prepared from a reaction mixture comprising a tetraethylammonium directing agent), which is disclosed in U.S. Pat. Nos. 3,766,093 and 3,894,104. Examples of suitable porous crystalline inorganic oxide materials having the defined X-ray diffraction pattern include MCM-22, PSH-3, SSZ-25, MCM-36, MCM-49 or MCM-56. MCM-22 is described in U.S. Pat. No. 4,954,325, PSH-3 is described in U.S. Pat. No. 4,439,409, SSZ-25 is described in U.S. Pat. No. 4,826,667, MCM-36 is described in U.S. Pat. No. 5,250,277, MCM-49 is described in U.S. Pat. No. 5,236,575 and MCM-56 is described in U.S. Pat. No. 5,362,697. The entire contents of each of the aforementioned patents are incorporated herein by reference.

Typically, the first catalyst comprises at least 1 wt. %, preferably at least 10 wt. %, more preferably at least 50 wt. %, and most preferably at least 65 wt. %, of the first molecular sieve. The second catalyst comprises at least 1 wt. %, preferably at least 10 wt. %, more preferably at least 50 wt. %, and most preferably at least 65 wt. %, of the second molecular sieve.

The catalyst system has a weight ratio of the first catalyst over the second catalyst in the range of 5:95 to 75:25, preferably in the range of 10:90 to 60:40, and more preferably in the range of 20:80 to 50:50.

In some embodiments, the first molecular sieve has an Alpha value of at least 150, such as at least 200. In other embodiments, the first molecular sieve has an Alpha value in the range of 100-1500, preferably in the range of 200-500.

Where the first molecular sieve is MCM-49, the MCM-49 can have a composition involving the molar ratio YO₂ over X₂O₃=n, wherein X is a trivalent element, such as aluminum, boron, iron, indium and/or gallium, preferably aluminum; Y is a tetravalent element, such as silicon, tin and/or germanium, preferably silicon; and n is less than about 35, e.g. from 2 to less than about 35, usually from about 10 to less than about 35, more usually from about 15 to about 31. MCM-49 is more fully described in the prior art, particularly U.S. Pat. Nos. 5,236,575; 5,304,698; and U.S. Application Publication 20080116112.

In a preferred embodiment, the first molecular sieve is MCM-49 and the second molecular sieve is ZSM-12.

Where the second molecular sieve is ZSM-12, the ZSM-12 can have a composition involving the molar YO₂ over X₂O₃=n, wherein X is a trivalent element, such as aluminum, boron, iron, indium and/or gallium, preferably aluminum; Y is a tetravalent element, such as silicon, tin and/or germanium, preferably silicon; and n is less than 500, such as from 50 to less than 300. The ZSM-12 may further be selected so as to have an average crystal size of less than 0.1 micron, such as about 0.05 micron, and a Diffusion Parameter, D/r², for mesitylene of at least 1000×10⁻⁶ sec⁻¹, such as at least 2000×10⁻⁶ sec⁻¹, when measured at a temperature of 100° C. and a mesitylene pressure of 2 torr.

As used herein, the Diffusion Parameter of a particular porous crystalline material is defined as D/r²×10⁶, wherein D is the diffusion coefficient (cm²/sec) and r is the crystal radius (cm). The required diffusion parameters can be derived from sorption measurements provided the assumption is made that the plane sheet model describes the diffusion process.

Thus for a given sorbate loading Q, the value Q/Q_(∞), where Q_(∞) is the equilibrium sorbate loading, is mathematically related to (Dt/r²)^(1/2) where t is the time (sec) required to reach the sorbate loading Q. Graphical solutions for the plane sheet model are given by J. Crank in “The Mathematics of Diffusion”, Oxford University Press, Ely House, London, 1967.

In some embodiments, the second molecular sieve has an Alpha value of at least 20, such as at least 30. In other embodiments, the second molecular sieve has an Alpha value in the range of 20-500, preferably in the range of 20-100, alternatively in the range of 30-100.

The alpha value test is a measure of the cracking activity of a catalyst and is described in U.S. Pat. No. 3,354,078 and in the Journal of Catalysis, Vol. 4, p. 527 (1965); Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980), each incorporated herein by reference as to that description. The experimental conditions of the test used herein include a constant temperature of 538° C. and a variable flow rate as described in detail in the Journal of Catalysis, Vol. 61, p. 395.

It may be desirable to incorporate each molecular sieve in the catalyst composition with another material that is resistant to the temperatures and other conditions employed in the transalkylation process of the disclosure. Such materials include active and inactive materials and synthetic or naturally occurring zeolites, as well as inorganic materials such as clays, silica and/or metal oxides such as alumina. The inorganic material may be either naturally occurring, or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides.

Use of a material in conjunction with each molecular sieve, i.e. combined therewith or present during its synthesis, which itself is catalytically active, may change the conversion and/or selectivity of the catalyst composition. Inactive materials suitably serve as diluents to control the amount of conversion so that transalkylated products can be obtained in an economical and orderly manner without employing other means for controlling the rate of reaction. These catalytically active or inactive materials may be incorporated into, for example, naturally occurring clays, e.g. bentonite and kaolin, to improve the crush strength of the catalyst composition under commercial operating conditions. It is desirable to provide a catalyst composition having good crush strength because in commercial use, it is desirable to prevent the catalyst composition from breaking down into powder-like materials.

Naturally occurring clays that can be composited with each molecular sieve as a binder for the catalyst composition include the montmorillonite and kaolin family, which families include the subbentonites, and the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification.

In addition to the foregoing materials, each molecular sieve can be composited with a porous matrix binder material, such as an inorganic oxide selected from the group consisting of silica, alumina, zirconia, titania, thoria, beryllia, magnesia, and combinations thereof, such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. It may also be advantageous to provide at least a part of the foregoing porous matrix binder material in colloidal form so as to facilitate extrusion of the catalyst composition.

Each molecular sieve is usually admixed with the binder or matrix material so that the final catalyst composition contains the binder or matrix material in an amount ranging from 5 to 95 wt. %, and typically from 10 to 60 wt. %.

The first catalyst comprises 0.01 to 5 wt. %, preferably 0.1 to 2 wt. %, more preferably 0.1 to 1 wt. %, of a first metal element of Groups 6-10. The second catalyst comprises 0 to 5 wt. %, preferably 0.01 to 2 wt. %, more preferably 0.01 to 1 wt. %, of a second metal element of Groups 6-10. The first metal element and the second metal element may be at least one hydrogenation component, such as tungsten, vanadium, molybdenum, rhenium, chromium, manganese, a metal selected from Groups 6-10 of the Periodic Table of the Elements, or mixtures thereof. Specific examples of useful metals are iron, ruthenium, osmium, nickel, cobalt, rhodium, iridium, and noble metals such as platinum or palladium. Preferably, the hydrogenation component is palladium, platinum or rhenium.

The amount of the hydrogenation component is selected according to a balance between hydrogenation activity and catalytic functionality. Less of the hydrogenation component is required when the most active metals such as platinum are used as compared to palladium, which does not possess such strong hydrogenation activity. Generally, the catalyst composition contains less than 5 wt. % of the hydrogenation component and typically from 0.01 wt. % to 2 wt. % of the component.

The hydrogenation component can be incorporated into the catalyst composition by co-crystallization, exchanged into the composition to the extent a Group 13 element, e.g., aluminum, is in the molecular sieve structure, impregnated therein, or mixed with the molecular sieve and binder. Such component can be impregnated in or on the molecular sieve, for example in the case of platinum, by treating the molecular sieve with a solution containing a platinum metal-containing ion. Suitable platinum compounds for impregnating the catalyst with platinum include chloroplatinic acid, platinous chloride and various compounds containing the platinum ammine complex, such as Pt(NH₃)₄Cl₂.H₂O.

Alternatively, a compound of the hydrogenation component may be added to the molecular sieve when it is being composited with a binder, or after the molecular sieve and binder have been formed into particles by extrusion or pelletizing.

After treatment with the hydrogenation component, the molecular sieve is usually dried by heating at a temperature of 65° C. to 160° C., typically 110° C. to 143° C., for at least 1 minute and generally not longer than 24 hours, at pressures ranging from 100 to 200 kPa-a. Thereafter, the molecular sieve may be calcined in a stream of dry gas, such as air or nitrogen, at temperatures of from 260° C. to 650° C. for 1 to 20 hours. Calcination is typically conducted at pressures ranging from 100 to 300 kPa-a.

Prior to use, steam treatment of the catalyst composition may be employed to minimize the aromatic hydrogenation activity of the catalyst composition. In the steaming process, the catalyst composition is usually contacted with from 5 to 100% steam, at a temperature of at least 260° to 650° C. for at least one hour, specifically 1 to 20 hours, at a pressure of 100 to 2590 kPa-a.

In addition, prior to contacting the catalyst composition with the hydrocarbon feed, the hydrogenation component can be sulfided. This is conveniently accomplished by contacting the catalyst with a source of sulfur, such as hydrogen sulfide, at a temperature ranging from about 320 to 480° C. The source of sulfur can be contacted with the catalyst via a carrier gas, such as hydrogen or nitrogen. Sulfiding per se is known and sulfiding of the hydrogenation component can be accomplished without more than routine experimentation by one of ordinary skill in the art in possession of the present disclosure.

In some embodiments, this disclosure relates to an apparatus adapted for transalkylation of a C9+ feedstock comprising:

-   -   a. a reactor containing a first catalyst having a first         molecular sieve having a MWW framework type followed by a second         catalyst having a second molecular sieve having a Constraint         Index less than 3; and     -   b. means for contacting a C9+ feedstock and a C₆-C₇ feedstock to         the first catalyst under first conditions and then to the second         catalyst under second conditions.

In one aspect, the first conditions are the same as the second conditions. In another aspect, the first catalyst is loaded in a first reaction zone of the reactor and the second catalyst is loaded in a second reaction zone of the reactor.

In the apparatus of the disclosure, the first and second catalysts may be loaded in a same reactor, or may be loaded in two separate reactors. In all situations, the first catalyst is not mixed with the second catalyst and the hydrocarbon feedstocks and hydrogen are contacting with the first catalyst prior to contacting the second catalyst. In some embodiments, the first catalyst may be separated from the second catalyst by space or by inert materials, such as, alumina balls or sand. The means for contacting a C₉+ feedstock and a C₆-C₇ feedstock to the first catalyst under first conditions and then to the second catalyst under second conditions include:

-   -   (a) load the first catalyst on the top of the second catalyst         when the hydrocarbon feedstocks are flowing top-down;     -   (b) load the second catalyst on the top of the first catalyst         when the hydrocarbon feedstocks are flowing bottom-up;     -   (c) load the first catalyst in the inner part of the reactor and         the second catalyst outside of the first catalyst loading when         the hydrocarbon feedstocks are flowing inside-out; or     -   (d) load the second catalyst in the inner part of the reactor         and the first catalyst outside of the second catalyst loading         when the hydrocarbon feedstocks are flowing from outside to         inside.

Means for contacting a C₉+ feedstock and a C₆-C₇ feedstock to the first catalyst under first conditions and then to the second catalyst under second conditions include pipe arrangement, control valves, flow meters, pumps, or any combination thereof. Other means for contacting a C₉+ feedstock and a C₆-C₇ feedstock to the first catalyst under first conditions and then to the second catalyst under second conditions include pumping or supplying the C₉+ feedstock and C₆-C₇ feedstock to the catalyst and followed by pumping or supplying the product of the first contacting step to the second catalyst.

The aromatic feed used in the process of the disclosure comprises one or more aromatic compounds containing at least 9 carbon atoms. Specific C₉+ aromatic compounds found in a typical feed include mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), hemimellitene (1,2,4-trimethylbenzene), pseudocumene (1,2,4-trimethylbenzene), 1,2-methylethylbenzene, 1,3-methylethylbenzene, 1,4-methylethylbenzene, propyl-substituted benzenes, butyl-substituted benzenes, and dimethylethylbenzenes. Suitable sources of the C₉+ aromatics are any C₉+ fraction from any refinery process that is rich in aromatics. This aromatics fraction contains a substantial proportion of C₉+ aromatics, e.g., at least 80 wt. % C₉+ aromatics, wherein preferably at least 80 wt. %, and more preferably more than 90 wt. %, of the hydrocarbons will range from C₉ to C₁₂. Typical refinery fractions which may be useful include catalytic reformate, FCC naphtha or TCC naphtha.

The feed to the process of the disclosure also includes benzene or toluene. In one practical embodiment, the feed to the transalkylation reactor comprises C₉+ aromatics hydrocarbons and toluene. The feed may also include recycled/unreacted toluene and C₉+ aromatic feedstock that is obtained by distillation of the effluent product of the transalkylation reaction itself. Typically, toluene constitutes from 0 to 90 wt. %, such as from 10 to 70 wt. % of the entire feed, whereas the C₉+ aromatics component constitutes from 10 to 100 wt. %, such as from 30 to 85 wt. % of the entire feed to the transalkylation reaction.

The feedstock may be characterized by the methyl over single aromatic ring molar ratio. In some embodiments, the combined feedstock (the combination of the C₉+ and the C₆-C₇ aromatic feedstocks) has a methyl over single aromatic ring molar ratio in the range of from 0.5 to 4, preferably from 1 to 2.5, more preferably from 1.5 to 2.25. The methyl over single aromatic ring molar ratio may be adjusted by adjusting relative flowrate of the C₉+ and the C₆-C₇ aromatic feedstocks and/or the relative C₆/C₇ ratio of the C₆-C₇ aromatic feedstock.

In some embodiments, this disclosure relates to a process for producing xylene comprising:

-   -   a. contacting a C₉+ aromatic feedstock, hydrogen and a C₆-C₇         aromatic feedstock with a first catalyst comprising 0.01 to 5         wt. %, preferably 0.01 to 1 wt. %, of at least one source of a         first metal element of Groups 6-10 and a first molecular sieve         having a MWW framework under first conditions to form a first         product, wherein the first conditions are selected such that the         first product is substantially free of olefinic components and         the first product contains at least 50 wt. % less ethyl-aromatic         compounds and at least 75 wt. % less propyl-aromatic compounds         than the C₉+ aromatic feedstock; then     -   b. contacting at least a portion of the first product with a         second catalyst comprising 0 to 5 wt. %, preferably 0.01 to 1         wt. %, of at least one source of a second metal element of         Groups 6-10 and a second molecular sieve having a Constraint         Index less than 3 under second conditions, wherein the second         conditions are sufficient to transalkylate at least a portion of         the C₉+ aromatic compounds in the C₉+ aromatic feedstock with at         least a portion of the C₆-C₇ aromatic compounds in the C₆-C₇         aromatic feedstock to form a second product comprising xylene,         wherein the second conditions are selected such that the second         product is substantially free of olefinic components and the         xylene yield is in the range of 20 to 50 wt. %, and wherein the         second product contains at least 60 wt. %, preferably at least         65 wt. %, still more preferably at least 70 wt. % less         ethyl-aromatic compounds and at least 70 wt. %, preferably at         least 75 wt. %, still more preferably at least 85 wt. % less         propyl-aromatic compounds than the C₉+ aromatic feedstock; and     -   c. recovering the xylene.

In another embodiments, this disclosure relates to a process comprising:

-   -   a. contacting a C₉+ aromatic feedstock with a first catalyst         comprising 0.01 to 5 wt. %, preferably 0.01 to 1 wt. %. of at         least one source of a first metal element of Groups 6-10 and a         first molecular sieve having a MWW framework under first         conditions to form a first product, wherein the first conditions         are selected such that the first product is substantially free         of olefinic components and the first product contains at least         50 wt. % less ethyl-aromatic compounds and at least 75 wt. %         less propyl-aromatic compounds than the C₉+ aromatic feedstock;         then     -   b. contacting at least a portion of the first product with a         second catalyst comprising 0 to 5 wt. %, preferably 0.01 to 1         wt. %, of at least one source of a second metal element of         Groups 6-10 and a second molecular sieve having a Constraint         Index less than 3 under second conditions to form a second         product, wherein the second conditions are selected such that         the second product is substantially free of olefinic components         and the xylene yield is in the range of 20 to 50 wt. %, and         wherein the second product contains at least 60 wt. %,         preferably at least 65 wt. %, still more preferably at least 70         wt. % less ethyl-aromatic compounds and at least 70 wt. %,         preferably at lest 75 wt. %, still more preferably at least 85         wt. % less propyl-aromatic compounds than the C₉+ aromatic         feedstock.

The process can be conducted in any appropriate reactor including a radial flow, fixed bed, continuous down flow or fluid bed reactor. The first conditions and/or the second conditions comprise a temperature in the range of 100 to 1000° C., preferably in the range of 300 to 500° C.; a pressure in the range of 790 to 7000 kPa-a (kilo-Pascal absolute), preferably in the range of 2170 to 3000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, preferably in the range of 1-10; a WHSV in the range of 0.01 to 100 hr⁻¹, preferably in the range of 1-20. The second conditions comprise a temperature in the range of 100 to 1000° C., a pressure in the range of 790 to 7000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹.

The first and/or the second conditions may be same. The first and the second conditions are sufficient to convert the heavy aromatic feed to a product containing more xylene than the combined feedstock.

In some embodiments, the first conditions are selected such that the first product is substantially free of olefinic components and the first product contains at least 50 wt. % less, preferably at least 70 wt. % less, ethyl-aromatic compounds and at least 75 wt. % less, preferably at least 85 wt. % less, propyl-aromatic compounds than the C₉+ aromatic feedstock.

In other embodiments, the second conditions are sufficient to transalkylate at least a portion of the C₉+ aromatic compounds in the C₉+ aromatic feedstock with at least a portion of the C₆-C₇ aromatic compounds in the C₆-C₇ aromatic feedstock to form a second product comprising xylene, wherein the second conditions are selected such that the second product is substantially free of olefinic components and the xylene yield is in the range of 20 to 50 wt. %, and wherein the second product contains at least 70 wt. % less, preferably at least 80 wt. % less, ethyl-aromatic compounds and at least 85 wt. % less, preferably at least 95 wt. % less, propyl-aromatic compounds than the at least a portion of the first product.

In some embodiments, where the C₉+ aromatic feedstock and/or the C₆-C₇ aromatic feedstock contains paraffinic compounds, the process further comprises a step of contacting the paraffinic compounds in the C₉+ aromatic feedstock and/or the C₆-C₇ aromatic feedstock with a third catalyst comprising a third molecular sieve having a Constraint Index in the range of 3-12 under first cracking conditions sufficient to crack at least 50 wt. % of the paraffinic compounds.

In some embodiments, where the second product contains paraffinic compounds, the process further comprises a step of contacting the paraffinic compounds in the second product with a fourth catalyst comprising a fourth molecular sieve having a Constraint Index in the range of 3-12 under second cracking conditions sufficient to crack at least 50 wt. % of the paraffinic compounds in the second product.

The first cracking conditions and/or the second cracking conditions comprise a temperature in the range of 100 to 1000° C., preferably in the range of 300 to 500° C.; a pressure in the range of 790 to 7000 kPa-a (kilo-Pascal absolute), preferably in the range of 2170 to 3000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, preferably in the range of 1-10; a WHSV in the range of 0.01 to 100 hr⁻¹, preferably in the range of 1-20. The second conditions comprise a temperature in the range of 100° C. to 1000° C., a pressure in the range of 790 to 7000 kPa-a, preferably in the range of 2170 to 3000 kPa-a; a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹.

In other aspects, the first conditions and the second conditions are selected such that the total ring-loss of the process is in the range of 0 to 3 wt. %, preferably in the range of 0.5-1.5 wt. %.

Generally the invention concerns a dual bed system for the transalkylation of heavy aromatics with benzene and toluene, optionally with a third bed. The first catalyst bed comprises a zeolite having a MWW topology together with a metal function to allow for saturation of the olefins formed via the dealkylation reactions. The second catalyst bed comprises a zeolite having a 12 MR pore structure to allow for transalkylation of the bulkier C₉+ molecules to occur, e.g. zeolites having a ZSM-12 (MTW), mordenite (MOR), beta (BEA) or USY (FAU) topology. The catalyst in the second bed also contains a metal which may be the same or different to the metal on the catalyst in the first bed.

The invention will now be illustrated through the use of Examples. One of skill in the art will recognize that the examples are merely representative and that the invention may be practiced other than as specifically described herein.

In Example 1, the performance of the top bed catalyst for dealkylation of a heavy aromatics feed will be demonstrated. In Comparative Example 2, the performance of a single bed transalkylation catalyst based on ZSM-12 will be illustrated. In Example 3, the performance of the dual bed system of this invention will be illustrated using the catalysts of Example 1 and Comparative Example 2. In Example 4, the performance of the dual bed system of this invention will be illustrated using the catalyst of Example 1 and a mordenite-based second bed catalyst.

EXAMPLE 1 Dealkylation of Heavy Aromatics Feed Using MWW Zeolite

A catalyst composed of 80% MCM-49 zeolite and 20% alumina was extruded as a 1/20 inch quadrilobe. 0.1% Platinum was then added to this catalyst base via incipient wetness techniques known to those skilled in the art. This catalyst was then tested in a fixed-bed microunit. The reactor pressure was 350 psig and the H₂:HC ratio was 2:1. The feed to the reactor contained 85% heavy aromatics and 15% benzene+toluene. A detailed analysis of the feed is shown in Table 1. The catalyst was reduced in hydrogen for 1 hour at 400° C., and then sulfided with 400 wt. ppm H₂S in H₂, prior to the introduction of feed. The total amount of sulfur added to the reactor was 5 moles S per mole Pt prior to feed introduction, and a further 10 moles S per mole Pt after feed was introduced. The activity of the catalyst was determined as a function of reactor temperature spanning the range from 380 to 420° C. The total feed flowrate, expressed as grams feed per gram catalyst per hour (WHSV) was 13.3 hr⁻¹. Product analysis occurred using on-line GC-FID with a 60 m DB-WAX column.

TABLE 1 Feed Composition Feed Composition % C5-gas 0.00 Benzene 8.08 Toluene 6.79 Ethylbenzene 0.00 Xylene 0.18 EthylToluene 20.15 Trimethylbenzene 42.74 Propylbenzene 2.79 1,n-ethylxylene 9.25 Tetramethylbenzene 4.27 Other C10 aromatic 3.58 Other C11 aromatic 0.54 Other C12 aromatic 0.00 Indanes 1.09 Alkylindanes 0.19 Napthalene 0.03 Alkylnaphthalene 0.00 Unidentified 0.32 The analysis of the product gas as a function of temperature is shown in Table 2.

TABLE 2 Product Composition for Various Temperatures Temperature (° C.) 380 390 400 410 420 C5-gas 3.31 3.79 4.46 5.11 6.06 Benzene 6.70 6.72 6.89 7.29 8.34 Toluene 11.42 12.39 13.36 14.71 18.13 Ethylbenzene 2.75 2.62 2.36 2.05 1.72 Xylenes 4.57 5.54 6.55 7.85 10.69 EthylToluene 11.57 10.41 8.94 7.35 4.03 Trimethylbenzene 40.86 40.59 40.00 39.23 36.66 Propylbenzene 0.02 0.01 0.01 0.00 0.04 1,n-ethylxylene 9.14 8.89 8.63 8.14 5.79 Tetramethylbenzene 5.11 5.02 5.05 4.98 5.04 Other C10 aromatic 1.05 0.87 0.72 0.53 0.27 Other C11 aromatic 1.12 1.08 1.11 1.07 0.87 Other C12 aromatic 0.02 0.01 0.01 0.01 0.00 Indanes 1.09 0.98 0.88 0.77 0.93 Alkylindanes 0.41 0.39 0.42 0.38 0.41 Napthalene 0.07 0.06 0.07 0.07 0.11 Alkylnaphthalene 0.06 0.05 0.06 0.06 0.34 Heavies 0.01 0.00 0.00 0.00 0.23 Unidentified 0.37 0.34 0.31 0.27 0.29

Comparison of the feed and product compositions shows that the catalyst is effective for de-alkylating the feed without catalyzing transalkylation reactions to any significant extent. Conversion of ethyltoluenes varies from 42.6% at 380° C. to 80.0% at 420° C. Overall de-ethylation rates, including ethyltoluene, ethylxylene and diethyl aromatics varied from 29.5% at 380° C. to 66.6% at 420° C. Depropylation rates, of predominately cumene and n-propylbenzene, were ˜96-98% over the range of temperature studied. Production of heavy components, e.g. alkylindanes, naphthalenes, alkylnaphthalenes, etc, was almost negligible. These data illustrate the suitability of this catalyst for de-alkylating the heavy aromatics feed. This de-alkylated feed is now more suitable for transalkylation reactions in a second catalyst bed as the C₉₊:C₇+C₆ ratio has been decreased.

COMPARATIVE EXAMPLE 2

A ZSM-12 zeolite having a Si/Al₂ ratio of ˜200 and crystal dimensions of ˜0.1 μm was formulated into a 1/16 inch cylindrical extrudate using Versal 300 alumina such that the mass ratio of zeolite crystal to alumina was 1.86:1. 0.1 wt. % Platinum was added to the catalyst during preparation. This extrudate was converted into the acidic form and then steamed to moderate the catalyst activity. This catalyst was then tested in a fixed-bed microunit. The reactor pressure was 350 psig and the H₂:HC ratio was 2:1. The feed to the reactor contained 85% heavy aromatics and 15% benzene+toluene. A detailed analysis of the feed is shown in Table 1. The catalyst was initially reduced in hydrogen at 427° C., then sulfided with 400 wt. ppm H₂S in H₂ prior to the introduction of feed. The total amount of sulfur added to the reactor was 7 moles S per mole Pt prior to feed introduction, and a further 15 moles S per mole Pt after feed was introduced. The reactor temperature was set to maintain an overall C₉+C₁₀ conversion of ˜54.5±0.5%. The total feed flowrate, expressed as grams feed per gram catalyst per hour (WHSV) was 3 hr⁻¹. Product analysis occurred using on-line GC-FID with a 60 m DB-WAX column.

TABLE 3 Conversions and Yield Slate for Single Bed Transalkylation at 3 WHSV Day-on-stream 15.5 Average Rxtr Temp (° F.) 780 Catalyst Performance De-ethylation (%) 49.1 De-propylation (%) 92.6 Conversion C9 + C10 54.5 Ethyltoluene 66.3 Ethylxylene 43.8 Yields (wt. %) Light Gas (C5−) 7.0 Benzene 3.2 Toluene 16.5 Ethylbenzene 2.2 Xylenes 30.2 C9 Aromatics 28.1 C10 Aromatics 10.4 Naphthalene 0.3 218+° C. b.p. material 2.8

The aging rate, expressed as the increase in temperature required to maintain constant conversion was 7.8° C./month for this catalyst.

EXAMPLE 3 Dual Bed Heavy Aromatics Transalkylation

The transalkylation of heavy aromatics with benzene and toluene was demonstrated over the dual bed catalyst system of this invention in a fixed-bed microunit. The top bed was a 1/20″ quadrilobe 80:20 MCM-49:Al₂O₃ extrudate that had been impregnated with 0.1% Pt, the same catalyst tested in Example 1. The bottom bed was a 1/16″ cylindrical 65:35 ZSM-12:Al₂O₃ extrudate containing 0.1 wt % Pt, as used in Comparative Example 2. The ratio of the top bed to the bottom bed was 3:7. The reactor pressure was 350 psig and the H₂:HC ratio was 2:1. The feed to the reactor contained 85% heavy aromatics and 15% benzene+toluene. A detailed analysis of the feed is shown in Table 1. The reactor temperature was varied between 390° C. and 420°, and the weight hourly space velocity (WHSV) was 4 hr⁻¹. On start-up, the catalyst beds were reduced in hydrogen at 420° C., and then sulfided with 400 wt. ppm H₂S in H₂ prior to introducing feed. The total amount of sulfur added to the reactor was 5 moles S per mole Pt prior to feed introduction, and a further 10 moles S per mole Pt after feed was introduced. After feed was introduced, the catalyst beds were de-edged by running with a H₂:HC ratio of 1 for the first 24 hours. Product analysis occurred using on-line GC-FID with a 60 m DB-WAX column.

TABLE 4 Conversion and Yield Slate for Dual Bed System at 4 WHSV Day-on-stream 4.7 Average Rxtr Temp (° F.) 788 Catalyst Performance De-ethylation (%) 69.3 De-propylation (%) 98.7 Conversion (%) C9 + C10 53.5 Ethyltoluene 76.9 Ethylxylene 52.8 Yields (wt. %) Light Gas (C5−) 6.9 Benzene 3.9 Toluene 16.6 Ethylbenzene 1.4 Xylenes 28.0 C9 Aromatics 28.4 C10 Aromatics 10.2 Naphthalene 0.2 218+° C. b.p. material 2.9

As the data in Table 4 indicates, the dual bed catalyst system has higher overall dealkylation rates than the single bed system, despite the fact that a higher space velocity is being used. As a result, significantly lower aging rates, when compared to the single bed comparative example, could be expected.

EXAMPLE 4 Dual Bed Heavy Aromatics Transalkylation

The transalkylation of heavy aromatics with benzene and toluene was demonstrated over the dual bed catalyst system of this invention in a fixed-bed microunit. The top bed was a 1/20 inch quadrilobe 80:20 MCM-49:Al₂O₃ extrudate that had been impregnated with 0.1% Pt, the same catalyst tested in Example 1. The bottom bed was a 1/16″ cylindrical 65:35 Mordenite:Al₂O₃ extrudate containing 0.1 wt % Pt. The bottom bed catalyst had been steamed to an alpha activity roughly similar to that of the ZSM-12 based catalyst tested in Example 3. The ratio of the top bed to the bottom bed was 3:7. The reactor pressure was 350 psig and the H₂:HC ratio was 2:1. The feed to the reactor contained 85% heavy aromatics and 15% benzene+toluene. A detailed analysis of the feed is shown in Table 1. The reactor temperature was varied between 390° C. and 420°, and the weight hourly space velocity (WHSV) was 4 hr⁻¹. On start-up, the catalyst beds were reduced in hydrogen at 420° C., and then sulfided with 400 wt. ppm H₂S in H₂ prior to introducing feed. The total amount of sulfur added to the reactor was 5 moles S per mole Pt prior to feed introduction, and a further 10 moles S per mole Pt after feed was introduced. After feed was introduced, the catalyst beds were de-edged by running with a H₂:HC ratio of 1 for the first 24 hours. Product analysis occurred using on-line GC-FID with a 60 m DB-WAX column.

TABLE 5 Conversion and Yield Slate for Dual Bed System at 4 WHSV Day-on-stream 4.7 Average Rxtr Temp (° F.) 788 Catalyst Performance De-ethylation (%) 79.4 De-propylation (%) 99.1 Conversion C9 + C10 55.7 Ethyltoluene 82.9 Ethylxylene 71.6 Yields (wt. %) Light Gas (C5−) 7.9 Benzene 4.1 Toluene 16.7 Ethylbenzene 1.2 Xylenes 27.6 C9 Aromatics 28.4 C10 Aromatics 8.3 Naphthalene 0.2 218+° C. b.p. material 3.1

As the data in Table 5 indicates, the dual bed catalyst system has high overall dealkylation rates, with de-ethylation rates approaching 80%. With this catalyst system, the amount of heavy components, specifically the aromatics boiling at temperatures greater than 218° C., is slightly higher than that compared to the dual bed system using ZSM-12 in the second bed. Overall, the data in Table 5 indicates that catalysts based on mordenite are also suitable transalkylation catalysts that can be used in the dual bed system that uses MWW-based top bed catalysts.

As has been indicated by this disclosure, the use of a dual bed system for heavy aromatics transalkylation provides significant benefits over existing technologies. The dual bed system allows for processing of heavier feeds, and for processing feeds at higher throughput through the reactor, thereby increasing production. In addition, the low aging rate of this dual bed system allows for longer catalyst cycles, and the reduced down-time resulting from reduced catalyst change-outs or catalyst regenerations leads to significant monetary savings.

These examples serve to illustrate the invention, but should in no way limit the scope of the invention. It is envisioned that a multitude of catalysts having a 12 MR pore structure would be suitable for the second bed. Examples of other catalysts suitable for the second bed include zeolite beta, USY, NU-87, and combinations of zeolites, including combinations of 10 MR zeolites with 12 MR zeolites. These zeolites may employ a hydrogenating metal as well.

As a further embodiment of this invention, a third bed may be used in conjunction with the dual beds already discussed. The purpose of this third bed is to catalytically crack non-aromatic species, e.g. methylcyclohexane, that may form as a result of hydrogenation of aromatic species in the first two beds. These non-aromatic species often have boiling points in the same range as benzene, and are therefore difficult to separate by distillation, making their removal via catalytic cracking desirable. The use of this third bed, often making up less than 15% of the total catalyst mass, is optional, and would generally only be employed to meet benzene purity specifications.

The meanings of terms used herein shall take their ordinary meaning in the art; reference shall be taken, in particular, to Handbook of Petroleum Refining Processes, Third Edition, Robert A. Meyers, Editor, McGraw-Hill (2004). In addition, all patents and patent applications, test procedures (such as ASTM methods), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted. Also, when numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. Note further that Trade Names used herein are indicated by a ™ symbol or ® symbol, indicating that the names may be protected by certain trademark rights, e.g., they may be registered trademarks in various jurisdictions.

While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains. 

1. A process to produce a product containing xylenes comprising contacting a C₉₊ aromatic feedstock, hydrogen, and a C₆-C₇ aromatic feedstock with a first catalyst comprising a first molecular sieve selected from the group consisting of MCM-22 and MCM-49 and 0.01 to 5 wt. % of a first metal element of Groups 6-10 under first conditions to form a first product, then contacting at least a portion of said first product with a second catalyst comprising a second molecular sieve selected from the group consisting of ZSM-12 and mordenite and 0 to 5 wt. % of a second metal element of Groups 6-10 and under second conditions to form a second product comprising xylenes.
 2. The process according to claim 1, further comprising contacting at least a portion of second product with a zeolite of the MFI topology.
 3. The process according to claim 1, further comprising a step of adjusting the flowrate of said C₉+ aromatic feedstock and the flowrate of said C₆-C₇ aromatic feedstock so that the combined aromatic feedstock has a methyl to single aromatic ring molar ratio in the range between 0.5 to
 4. 4. The process according to claim 1, wherein said first metal element and said second metal element are independently selected from Pt, Pd, Ir, Re, and mixtures thereof.
 5. The process according to claim 1, wherein said first conditions comprise a temperature in the range of 100° C. to 1000° C., a pressure in the range of 790-7000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹, and wherein said second conditions comprise a temperature in the range of 100° C. to 1000° C., a pressure in the range of 790-7000 kPa-a, a H₂:HC molar ratio in the range of 0.01 to 20, a WHSV in the range of 0.01 to 100 hr⁻¹.
 6. The process according to claim 1, wherein said first conditions and said second conditions are selected such that the total ring-loss of the process is in the range of 0 to 3 wt. %.
 7. The process according to claim 1, wherein said first molecular sieve is MCM-49 and said second molecular sieve is ZSM-12.
 8. The process according to claim 7, wherein said MCM-49 has a particle size of less 1 micron, and said ZSM-12 has a particle size of less than 0.5 micron.
 9. The process according to claim 1, wherein wherein said second catalyst comprises said second metal element and wherein the weight ratio of said first catalyst over said second catalyst is in the range of 5:95 to 75:25.
 10. The process according to claim 1, wherein said first catalyst comprises 0.01 to 1 wt. % said first metal element, and said second catalyst comprises 0.01 to 1 wt. % said second metal element.
 11. The process according to claim 1, wherein said first metal element and said second metal element are Pt.
 12. The process according to claim 1, wherein said first molecular sieve is MCM-49 and said second molecular sieve is ZSM-12, wherein said MCM-49 has a particle size of less 1 micron, and said ZSM-12 has a particle size of less than 0.5 micron, including a step of sulfiding at least one of said first and said second catalysts prior to said contacting step. 